Comparison of response of experimental tumors to adjuvant chemotherapy for minimal disease with the response of solid large tumors to similar drug regimens has indicated that for many tumors the small microscopic foci are more responsive. There are, however, a number of model tumors that respond far less to chemotherapy when applied to minimal disease than when applied to large tumors. The response of these tumors to adjuvant treatment shows similarities with the clinically observed response. The data do not support the hypothesis that the success of a drug combination given as adjuvant chemotherapy may be predicted from the success of the same treatment when applied for manifest disease.